1. Field of the Invention
The present invention relates to a reflective sensor for detecting the presence or absence of a subject to be detected at a detecting position, a filter used for the reflective sensor, and a method of detecting a subject to be detected using the filter.
2. Description of Related Art
There has been known a reflective sensor, including a light-emitting element for emitting light toward a detecting position, and a light-receiving element for receiving light emitted from the light-emitting element and reflected on a subject to be detected (for example, a sheet of paper) present at the detecting position and generating an electrical signal corresponding to a quantity of received light, used to detect the presence or absence of the subject to be detected at the detecting position based on the electrical signal from the light-receiving element.
The reflective sensor of this type has a risk that dust, such as paper particles generated from a sheet of paper as the subject to be detected, goes inside the reflective sensor. When dust goes inside the reflective sensor, the surfaces of the light-emitting element and the light-receiving element are contaminated, and these elements may become unable to emit or receive light in a satisfactory manner, which possibly results in false detection.
In order to eliminate this problem, there is known a reflective sensor that prevents entrance of dust by disposing a flat-shaped light-transmitting filter between the reflective sensor and the detecting position.
This arrangement, however, has a problem that part of light emitted from the light-emitting element is reflected on the surface of the filter to be incident on the light-receiving element, which makes precise detection of the presence or absence of the subject to be detected infeasible.